The ESR1 Mutations: From Bedside to Bench to Bedside

Abstract The ESR1 ligand–binding mutations were unveiled a number of years ago and are the most common genetic mechanism of acquired resistance to endocrine treatment, particularly, to aromatase inhibitors. The discovery of these mutations was enabled after advancements in sequencing technologies and when metastatic tissue samples were interrogated. The ESR1 ligand–binding domain mutations are activating mutations that lead to constitutive ligand-independent activity, which explains the emergence of these mutations under the selective pressure of aromatase inhibitors. Arnesen and colleagues have generated new models of the ESR1 mutations using CRISPR technology to generate single-cell–derived clones in which the ESR1 ligand–binding mutations were “knocked-in” and expressed under the endogenous promoter of estrogen receptor. The authors have extensively characterized these models and have shed new light on the functional consequences ESR1 mutations. See related article by Arnesen et al., p. 539